CCFL has been applied for the backlight sources of displays, especially for liquid crystal displays (LCDs), which requires a circuit to drive them. Due to the small size of conventional displays, so far one CCFL is enough for one display. However, recent displays tend to be enlarged in their scales, resulting in that two or more CCFLs are required for one backlight source, and there is thus a need of a driving circuit capable of driving two or more CCFLs for the backlight source.
A prior art driver to drive two CCFLs is proposed by U.S. Pat. No. 5,892,336 issued to Lin et al., which comprises a transformer having its primary side connected with an AC power supply and secondary side connected with two CCFLs. Since these two CCFLs are connected in series to the driver, the currents flowing through them are identical and therefore limit the CCFLs to be adjusted individually. However, there are always more or less variations between CCFLs once they are manufactured, and thus they have diversified luminescent features. As a result, this driver with CCFLs connected in series and thereby having the same driving current for all of the CCFLs cannot be available for applications that adjustable brightness of individual lamp is required.
An alternative driver proposed by U.S. Pat. No. 6,396,722 issued to Lin employs four MOS transistors and one transformer to form a full bridge circuit to drive a CCFL, yet this driver drives only one CCFL. Two independent full bridge circuits are required to drive two CCFLs individually, if this art is utilized. Even the driving currents are adjustable for respective CCFLs for their brightness to be uniformed when two independent full bridge circuits are provided, the cost and volumn of the driver are dramatically increased.
Therefore, it is desired a low-cost and small-size driver to adjust the driving current for individual CCFL in a multiple CCFL system to control their respective brightness.